Countersubject
A recurring counter-melody paired with a fugue subject.
What it means
A countersubject is a fixed melodic counter-line that regularly sounds together with the subject in a fugue, usually right after one voice has stated the subject and another takes it up in imitation.
Musical use
In a classical fugue, one voice first states the subject alone; once a second voice imitates it, the first voice continues with the countersubject, deliberately contrasting the subject in rhythm and contour so that both lines remain audible as independent strands. If the same countersubject, or a close variant, returns at every later entry of the subject, it is called an invertible or fixed countersubject; if a new counter-line appears at every entry instead, this is called free counterpoint. A countersubject differs from episode material, which stands between subject entries without stating the theme itself.
Performance practice
When listening to a fugue, follow the countersubject deliberately — it helps you spot each entry of the subject, since it usually appears right afterwards in another voice.